I was just referred an excellent web resource at work this week, the EKey Electronic Key for Identifying Freshwater Fish.
The ID Key is provided by the National Biological Information Infrastructure, or NBII (administered by the Biological Informatics Office of the U.S. Geological Survey ), and allows users to search freshwater fish ID database by taxon, dichotomous key, text-based search, and a shape and text-based search. The latter is an interesting and unique means of searching for fish, one that I have not encountered before. While not every species found throughout the US are listed, the database contains a very thorough and comprehensive species list. This is an excellent reference and web resource for assisting with fish identififcation.